It is generally recognized in the machining industry that problems relating to excessive vibrations, or "chatter", are among the major factors limiting machining productivity and surface finish quality of a machined workpiece. Chatter is caused by vibrations produced when the cutter of a machining or cutting system, given a particular rotating speed and feed rate, excites a resonance frequency in the system and workpiece. Vibrations are inherent in the machining process and can affect the surface finish of the workpiece. Excessive vibrations limit machining precision, and can be very destructive to a cutting tool and/or a workpiece.
The potential for workpiece chatter depends, at least in part, upon the hardness of the workpiece material. With the increased usage of hard materials such as super alloys, refractory alloys, and many of the titanium alloys, chatter has become a significant problem. The machining of these materials requires much machine operator attention and special care. The machinist must anticipate the possibility for chatter in a set-up phase and attempt to reduce chatter onset as much as possible by applying rigid fixturing. Beyond this procedure, there are a paucity of commercially available means for reducing chatter. Moreover, in practice it often occurs that by the time the machinist hears audible chatter and responds to it, damage may have already occurred in the form of excessive cutter wear (with an associated loss of tolerance of the machined surface due to a worn or broken cutter), poor or unacceptable workpiece surface finish, and increased wear of machine tool bearings and components. Additionally, high-pitched sounds associated with chatter can be annoying and possibly harmful to the operator.
Methods that are presently known in the art for controlling chatter are based upon procedures for controlling the machining process. Automatic chatter control systems which, by example, adjust machining parameters such as spindle (cutter) speed and/or feed rates have been employed to control chatter. Such systems, however, must be designed specifically for, and built into, the particular machine tool of interest.
U.S. Pat. No. 3,671,840, issued to Meyer et al., discloses an adaptive control for a turning machine. Vibrations of a tool or workpiece during a machining operation are measured to produce signals corresponding to actual vibrations in mutually perpendicular directions. The vibration signals are compared with signals corresponding to reference magnitudes in both directions. The results of the comparisons are used to correct feeding of the tool or workpiece in accordance with the comparison results.
U.S. Pat. No. 3,967,515, issued to Nachtigal et al., discloses an apparatus for controlling vibrational chatter in a machine tool utilizing a synthesis circuit that updates workpiece and machine tool positions. A cutting tool mounted on a machine structure is moved into contact with a workpiece which is mounted on the machine structure. A resultant cutting force is reflected back through the tool and machine structure and is then measured by a transducer, which generates a signal indicative thereof. This force signal is applied to the synthesis circuit for continuous calculation of the workpiece displacement in accordance with continuously updated static and dynamic characteristics of the machine structure and the workpiece. Simultaneously, an accelerometer detects the acceleration of one of the cutting tool and workpiece. A signal representative of actual horizontal acceleration of the tool into or away from the workpiece is used to derive a signal representing a corresponding actual horizontal displacement of the tool. An output signal of the synthesis circuit is compared with the horizontal displacement signal. When the two signals are not equal, the difference is used to generate an error signal for controlling a compensatory force actuator that is mounted on the machine structure. The compensatory force actuator comprises a bidirectional, force-delivering assembly having a housing containing a force actuator. The compensatory force actuator mechanically applies compensatory forces to the machine structure to supplement the applied cutting force and thereby adjust the tool displacement to offset and eliminate vibrational chatter.
Another method that is known in the art for controlling chatter is based upon manual control of the machining process. By example, manually operated machines rely upon the machine tool operator to alter the machining parameters once chatter occurs. Cutter speed and feed rates are the two most common parameters that the machinist adjusts when attempting to eliminate chatter. Traditionally, while initially setting up the machine tool assembly, the operator sets the cutter speed, depth and width of cut, and workpiece feed rate for a particular machining pass based upon the operator's experience and finished part requirements. Thereafter, during the machining process, the operator must stop the machine and adjust the feed rate and/or the spindle speed, or remove less material per machining pass in order to reduce chatter. Unfortunately, the need for constant operator vigilance and the time required for the operator to make such parameter adjustments slows the machining process. Moreover, although the chatter is likely to be reduced after the operator has made such parameter adjustments, damage may have already occurred to the workpiece and/or machining tool owing to the chatter that occurred before the operator intervention.
It can be appreciated that chatter reduces the efficiency of the machining process and reduces the quality of workmanship attained. A reduction of chatter in machining processes thus provides for increased machining efficiency and reductions in machining costs.